Multicellular organisms are organisms that consist of more than one cell, in contrast to single-celled organisms. To form a multicellular organism, these cells need to identify and attach to the other cells.

Multicellularity has evolved independently dozens of times in the history of Earth, for example once for plants, once for animals, once for brown algae, but perhaps several times for fungi, slime molds, and red algae. Multicellularity exists in both prokaryotes and eukaryotes, and first appeared several billion years ago in cyanobacteria. In order to reproduce, true multicellular organisms must solve the problem of regenerating a whole organism from germ cells (i.e. sperm and egg cells), an issue that is studied in developmental biology. Therefore, the development of sexual reproduction in unicellular organisms during the Mesoproterozoic is thought to have precipitated the development and rise of multicellular life.Multicellular organisms, especially long-living animals, also face the challenge of cancer, which occurs when cells fail to regulate their growth within the normal program of development. Changes in tissue morphology can be observed during this process.

Multicellularity allows an organism to exceed the size limits normally imposed by diffusion, conferring the competitive advantages of an increase in size. It also permits increasing complexity by allowing the differentiation of numerous cellular lineages within an organism.

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